Improvements in or relating to centralisers

ABSTRACT

A centraliser comprises longitudinally spaced collars connected by a plurality of springs, each of the springs comprising two or more bow sections.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a U.S. National Phase Application pursuant to35 U.S.C. §371 of International Application No. PCT/GB2015/052492 filedAug. 27, 2015, which claims priority to GB Patent Application No.1415115.3, filed Aug. 27, 2014 and GB Patent Application No. 1500789.1,filed Jan. 18, 2015. The entire disclosure contents of theseapplications are herewith incorporated by reference into the presentapplication.

BACKGROUND

Centralisers are used in the oil, gas and water well drilling industriesto centre a tubular member within a borehole (wellbore) or inside apreviously installed larger tubular member (casing).

The purpose of a centraliser is to facilitate running casing to thedesired depth and to assist in centring the casing in the wellbore. Oneof the main objectives of centralising a casing string is to facilitategood cement sealing within the annulus of the well bore between theouter surface of the casing and the formation wall, thereby isolatingfluids from different zones.

A centraliser is a mechanical device that keeps casing from contactingthe wellbore wall; maintaining a continuous 360-degree annular spacearound casing allows cement to completely seal the casing to theborehole wall.

There are two distinct classes of centralisers.

The older and more common is a simple, low-cost bow-spring design. Sincethe bow springs are slightly larger than the wellbore, they can providecomplete centralisation in vertical or slightly deviated wells. However,they often struggle to support the weight of the casing adequately indeviated or horizontal wellbores.

The second type is a rigid blade design. This type is rugged and workswell even in deviated or horizontal wellbores, but since thecentralisers are smaller than the wellbore, they will not provide asgood centralisation as bow-spring type centralizers in vertical wells.Rigid-blade casing centralisers are slightly more expensive and cancause trouble downhole if the wellbore is not in excellent condition.They are unable to compress and thereby unable to pass through narrowopenings that may be caused when the wellbore collapses. This may resultin expensive and time consuming operations to open up the hole in orderto enable passage of the rigid blade centralizer.

Effective centralisation assists in the removal of drilling mud from thewell bore annulus and helps ensure an even cement coat around thecasing. Poor centralising of the casing within the wellbore will lead toinadequate cementing and will result in costly repair or potentiallyeven ‘killing’ and losing the well.

In recent years the trend has been towards drilling more extended reachhorizontal wells and so there is a real need to develop centraliserswith a low moving force that can also support the weight of the casingand enable a good cement seal, whilst having the ability to pass throughtight spots.

The ability to support the casing through larger openings is often alsoa benefit. Eccentric casing can lead to unequal or unbalanced annuli onthe high and low side of the borehole resulting in mud pockets on thelow side because the cement will tend to follow the path of leastresistance.

In summary, centralisers assist in the efficient installation of casingand enable cementing of the casing within the borehole.

There are many existing patents for differing designs of centralisers,and basic designs have existed for over seventy years. More recentpatents often pertain to either the specific manufacturing process ormaterials used. The bow-spring centraliser is a well-establishedconcept, and whilst prior art covers an extensive array of geometricdesigns variations, the fundamental principles of the form and shaperemain largely the same.

Examples of bow-spring patents include U.S. Pat. No. 6,997,254, U.S.Pat. No. 8,196,670, U.S. Pat. No. 3,312,285, U.S. Pat. No. 2,228,648 andU.S. Pat. No. 4,909,322.

The present invention seeks to provide improvements in or relating tocentralisers.

A good centraliser for use in deviated or extended reach horizontalwells should be optimised to have a low moving force (minimum effortneeded to push the casing through the well), a high restoring force(maximum support given to weight of the casing), the ability to passthrough tight spots in collapsed or constricted boreholes (minimum outerdiameter upon full compression), ability to increase diameter whilstpassing through ‘under-reamed’ or ‘washout’ sections of the borehole(sections with a larger opening than intended), the ability to enableeven and full flow of cement through and around the centralizer so asnot to create voids or mud pockets that would require expensive remedialaction.

SUMMARY

In one aspect the present invention provides a centraliser comprising aplurality of springs, each spring comprising a larger, taller outwardbow and a smaller, shorter outward bow, and an intermediate inward bowbetween the outward bows.

The present invention also provides a centraliser for keeping wellborecasing from contacting a formation wall, the centraliser comprising aplurality of springs, each spring comprising a larger, taller outwardbow and a smaller, shorter outward bow, and an intermediate inward bowbetween the outward bows, the springs having a three phase action: i)whilst subject to low forces each spring acts as a larger, single bowspring against the formation wall; ii) with increased forces the inwardbow contacts the casing to transform the spring into a shorter, stifferbow spring; and iii) greater still forces cause the height of the twooutward bows to equalise, transforming the spring into two smaller andstiffer bow springs.

According to a further aspect of the present invention there is provideda centraliser for keeping wellbore casing from contacting a formationwall, the centraliser comprising two longitudinally spaced collarsconnected by a plurality of springs, the springs comprising means forreducing pressure on the formation within the well by spreading the loadover a greater contact area between the centraliser and the formationwall.

The present invention also provides a centraliser for keeping wellborecasing from contacting a formation wall, the centraliser comprising twolongitudinally spaced collars connected by a plurality of bow springs,the springs comprising means for reducing pressure on the formationwithin the well by spreading the load over a greater contact areabetween the centraliser and the formation wall, in which thecross-section of at least part of each box is curved.

Each bow may include a region of increased width. The cross section ofthe region of increased width may be curved. The region of increasedwidth may be provided in the region of an apex of the bow.

This provides a stronger bow that does not deform easily, as well asproviding a “ski” effect circumferentially as well as longitudinallyacross the formation i.e. there is no gouging and no cutting into theformation along the edge of the bow (which might otherwise occur if thebow apex were flat and had only two points of contact on thepredominantly round bore hole).

The present invention also provides a centraliser comprisinglongitudinally spaced collars connected by a plurality of springs, eachof the springs comprising two or more outward bow sections. The bows maybe generally the same or different; for example the same or a differentlength and/or the same or a different resting height (for example radialextent) in use.

The present invention also provides a centraliser comprising twolongitudinally spaced collars connected by a plurality of springs, eachof the springs comprising two or more curved sections, in which the, orat least two, of the sections have a different height in a restingposition.

The present invention also provides a centraliser comprising twolongitudinally spaced collars connected by a plurality of springs, eachof the springs having an undulating section.

The present invention also provides a centraliser comprising twolongitudinally spaced collars connected by a plurality of springs, eachof the springs comprises an irregular curve.

The present invention also provides a centraliser comprising twolongitudinally spaced collars connected by a plurality of springs, eachof the springs comprises one or more apices which contact a formation inuse, the or each apex region being wider than the remainder of thespring.

The present invention also provides a centraliser comprising twolongitudinally spaced collars connected by a plurality of springs, thesprings contacting the collars at junctions, in which the junctions arerounded whereby to reduce stress.

The present invention also provides a centraliser comprising twolongitudinally spaced collars connected by a plurality of springs, eachof the springs having a peak, in which the longitudinal position ofpeaks on at least two springs is different.

In some embodiments alternate springs have different peak positions. Forexample there may be two spring peak positions which are longitudinallyoffset from each other.

In some embodiments the peaks are spaced circumferentially and/orrotationally and/or longitudinally. For example, in one embodiment sixsprings are provided, giving a first set of three points of contactwhich are 120° apart and a second set of three points of contact whichare also 120° apart, but are also rotated 60° from the first set and arelongitudinally offset from the first set.

The present invention also provides a centraliser comprising twolongitudinally spaced collars connected by a plurality of springs, eachof the springs being generally helical. The or each spring may be acoiled spring.

A single helix that performs one complete revolution at the peakdiameter may be provided. This may have significant advantages duringmanufacturing and in operation—one part, no interconnections, less to gowrong or break.

The diameter of the helices may change along the length thereof. Forexample the diameter of the helices may decrease from a mid-pointtowards the collars.

The present invention also provides a centraliser comprising twolongitudinally spaced collars connected by a plurality of springs, eachof the springs being generally spiral. The or each spring may be acoiled spring.

The spring/s may have a generally circular cross-section. In someembodiments the cross section of spring/s may be polygonal, square,rectangular, hexagonal, diamond, elliptical, oval, egg-shape ortrapezoidal. In embodiments with a non-circular cross-section thesection may twist with the helix.

A circular cross-section may be preferred (for example because of easeof manufacture as well as inherent coil spring forces), but it may bebeneficial to use another shape cross-section e.g. for frictionreduction, for area spread, or maybe for manufacture (end collar twistsmay be better with flat edges).

Where a plurality of springs are provided all of the springs may havethe same cross section; in some embodiments one or more different crosssections are present in a spring array.

The present invention also provides a centraliser comprising twolongitudinally spaced collars connected by a plurality of springs, inwhich there is provided means for reduction of the co-efficient offriction of centraliser surfaces in contact with a formation and/or incontact with an casing outer wall.

In some embodiments at least part of the centraliser has a low-frictioncoating, for example

Molybdenum Disulphide or Diamond Like Carbon.

The centraliser may be provided with one or more low-friction pads.

The centraliser may be provided with one or more wear pads. The wear padmay be a sacrificial pad to protect the spring/s from damage, enablingit to reach its final resting place fully intact.

One or more low friction and/or wear pads may be fitted to a bow apexwhere they come into contact with the casing/formation. Alternatively oradditionally they may be fitted to the ends (for example the frontedges) of the end collars to protect against abrupt impacts ofconstrictions in a wellbore.

The centraliser may include surfaces with surface formations forreducing frictions. For example 3-dimensional patterns, a patternedsurface, protruding dimples or bosses.

The surface formations may be formed on the surface by, for example,impressing, cutting, coating or moulding.

The surface formations may be provided externally on wall formationsurface-contacting regions of the centraliser. Additionally oralternatively formations may be provided internally on casing/pipecontacting regions. In some embodiments surface formations are providedover substantially the entirety of the or each spring. In otherembodiments formations are provided only in certain regions. Forexample, the surface formations may be provided only on the springs; orjust spring peaks where present.

The present invention also provides a centraliser comprising one or morebows, the or each bow having a curved cross-section, and the or each bowhaving scalloped or wavy or ribbed edges for reducing the stressincurred as the bow flexes in use, whilst at the same time maintainingthe overall contact area for skiing across a formation.

The present invention also provides a centraliser comprising a pluralityof bow springs, each bow curving back in on itself at the two opposingends to form collars that are directly underneath the bow.

The present invention also provides a centraliser comprising a pluralityof bows, in which each bow loops around at both ends of arch/profilethereby creating a tube/hollow that in turn provides a pivot pointenabling an axle to pass through and hence permitting free rotation ofthe bow ends.

The axle might be round bar in a ring to form the end collar of thecentraliser assembly. This might in turn will be affixed to atraditional flat collar in order to maintain rigidity and preventtwisting and potential locking onto the pipe as the bows compress andstretch independently.

The present invention also provides a centraliser comprising one or morecoiled springs with a diameter of the helix greater in the middle thanat the two ends, and with a one or more revolutions of helix of equaldiameter at the ends, and with one or more revolution at the centre atthe largest diameter.

The present invention also provides a centraliser comprising two or morecoiled springs with diameter of the helix greater in the middle than atthe two ends, and with a partial revolution or one or more (full orpartial) revolutions of helix of equal diameter at the ends.

The pitch of the helix may be such that the plurality of helices areable to intertwine.

The coils at the ends may be welded together so as to create onecentraliser assembly and maintain interaction between all helices. Thismay be similar to other helix designs herein with multiple helices, butwith coiled ends, intertwined and simply welded together.

End collar coils may be slotted into tubular sections which in turn arewelded onto an inner collar.

End collar coils may be bent twice to create an axial parallel sectionbefore returning to the original helical angle, thereby producing acircumferential mechanical stop for the next helix in the sequence.Alternatively the return may be such that the remainder of the bar is at90 deg to the axial direction, i.e. in the radial plane (this is what isdrawn on the sketches. Whilst a return to the helical path may work, andmay be employed, it may be preferable for prevention of dislodging ifthe return is in the radial plane, or maybe even better if it doublesback toward the centraliser slightly so that the end of the bar lodgesinto the kink of the next helix.

Two or more helices may be provided. For example three helices may beprovided and interlocked by ensuring that the stop is set approximately120 degrees from the end of the next helix and having the three helicesinterlock both circumferentially as well as axially. If, for example,four helices are provided then the “stop” will be set to the appropriateangle, in this case 90 deg.

The present invention also provides a centraliser comprisinglongitudinally spaced collars connected by a plurality of springs, eachof the springs comprising two or more bow sections, the transitionbetween the bow and the end collar consisting of two bends of opposingdirection to create an S-bend.

The present invention also provides a centraliser comprising one or morecoiled springs with a plurality of sections differing in pitch anddiameter such that a transition is made from an end collar section to acentralising bow section to a mid-collar to another bow section and thento an opposite end collar.

There may be two or more mid-collar sections. There may be two or morebow sections.

Each bow section may be offset in angle such that the resulting peaksare evenly spaced around the circle when viewed from the end axialorientation.

Each bow section may have a partial revolution so that the accumulativetotal revolution of all the bow sections is at least one full revolutionof the circle.

The present invention also provides a bow spring centraliser comprisinga plurality of bow springs and two internally facing end collars.

A small radius curve/bend may be provided between each bow and theinternally facing end collars which provides sufficient flexibility andmovement around the transition area such that when the bow is compressedand loaded the stresses are distributed through the curve/bend anddissipated away from the transition radius, effectively allowing the bowto flex (almost pivot) somewhat rather than to crease at the transitionarea.

The present invention also provides a bow spring centraliser comprisinga plurality of bow springs and two externally facing end collars.

A small radius curve/bend may be provided between each bow and theexternally facing end collars which provides sufficient flexibility andmovement around the transition area such that when the bow is compressedand loaded the stresses are distributed through the curve/bend anddissipated away from the transition radius, effectively allowing the bowto flex (almost pivot) somewhat rather than to crease at the transitionarea.

The present invention also provides a centraliser having an end collarwith inward facing castellations.

The present invention also provides a centraliser with a wavy orscalloped end collar.

The present invention also provides a centraliser with a twin phase bowdesign, the centraliser having a plurality of major bow springsextending between end collars, each major spring having a respectiveminor spring, the minor springs being positioned under the majorsprings, in a first loading phase just the major bow springs arecompressed and in a second, greater loading phase the major bow springsare pressed onto their respective minor springs.

The end collars may be inward facing collars.

The present invention also provides a well bore having one or morecentralisers as described herein.

Aspects and embodiments of the present invention aim to improvecentraliser design and performance to achieve greater reliability, forexample in extended reach wells.

Different aspects of the present invention may be used separately ortogether.

Further particular and preferred aspects of the present invention areset out in the accompanying independent and dependent claims. Featuresof the independent and dependent claims may be combined with thefeatures of the other independent and dependent claims as appropriate,and in combination other than those explicitly set out in the claims.

BRIEF DESCRIPTION OF THE FIGURES

The present invention will now be more particularly described, by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1.1 is an isometric view of a centraliser formed in accordance withthe present invention;

FIG. 1.2 is a side elevation of the centraliser of FIG. 1.1;

FIG. 1.3 is an end view of the centraliser of FIG. 1.1;

FIG. 2 is an isometric view of a centraliser formed according to analternative aspect;

FIG. 2.1 shows three phases of operation of a spring forming part of thecentraliser of FIG. 2;

FIG. 2.2 illustrates bow-spring stiffness during the three phases shownin FIG. 2.1;

FIG. 3 is an isometric view of a centraliser formed according to afurther aspect;

FIG. 4.1 is an isometric view of a single helix spring coil centraliserformed according to the present invention;

FIG. 4.2 is a side view of the centraliser of FIG. 4.1;

FIG. 4.3 is an end view of the centraliser of FIG. 4.1;

FIG. 4.4 is a further side view of the centraliser of FIG. 4.2, shownrotated 90 degrees;

FIG. 5.1 is an isometric view of a single helix spring coil centraliserformed according to a further aspect;

FIG. 5.2 is a magnified view of one end of the centraliser of FIG. 5.1;

FIG. 6.1 is an isometric view of a single helix spring coil centraliserformed according to a further aspect;

FIG. 6.2 is a magnified view of one end of the centraliser of FIG. 6.1;

FIG. 7.1 is an isometric view of a single helix spring coil centraliserformed according to a further aspect;

FIG. 7.2 is a magnified view of one end of the centraliser of FIG. 7.1;

FIG. 8.1 is an isometric view of a centraliser formed according to analternative aspect;

FIG. 8.2 is a side view of the centraliser of FIG. 8.1;

FIG. 8.3 is an end view of the centraliser of FIG. 8.1 with curvatureshown exaggerated to illustrate the curvature;

FIG. 8.4 is a magnified view of one bow of the centraliser of FIG. 8.3with curvature shown exaggerated to illustrate the curvature;

FIG. 8.5 is a further side view of the centraliser of FIG. 8.2, shownrotated 90 degrees;

FIG. 8.6 is a magnified view of one end of the centraliser of FIG. 8.1illustrating rounded corners;

FIG. 9.1 is an isometric view of a centraliser formed according to afurther aspect;

FIG. 9.2 is a side view of the centraliser of FIG. 9.1;

FIG. 9.3 is an end view of the centraliser of FIG. 9.1;

FIG. 9.4 is a further side view of the centraliser of FIG. 9.2, shownrotated 90 degrees;

FIG. 10.1 is an isometric view of a centraliser formed according to afurther aspect;

FIG. 10.2 is a side view of the centraliser of FIG. 10.1;

FIG. 10.3 is an end view of the centraliser of FIG. 10.1;

FIG. 10.4 is a magnified view of one bow of the centraliser of FIG.10.3;

FIG. 10.5 is a further side view of the centraliser of FIG. 10.2, shownrotated 90 degrees;

FIG. 11.1 is an isometric view of a centraliser formed according to afurther aspect;

FIG. 11.2 is an end view of the centraliser of FIG. 11.1;

FIG. 11.3 is a magnified view of one bow of the centraliser of FIG.11.2;

FIG. 11.4 is a perspective view of the view of FIG. 11.3;

FIG. 12.1 is an isometric view of a single helix spring coil centraliserformed according to the present invention;

FIG. 12.2 is a side view of the centraliser of FIG. 12.1;

FIG. 12.3 is an end view of the centraliser of FIG. 12.1;

FIG. 12.4 is a further side view of the centraliser of FIG. 12.2, shownrotated 90 degrees;

FIG. 13.1 is an isometric view of a multiple helix spring coilcentraliser formed according to a further aspect;

FIG. 13.2 is a side view of the centraliser of FIG. 13.1;

FIG. 13.3 is an end view of the centraliser of FIG. 13.1;

FIG. 13.4 is a further side view of the centraliser of FIG. 13.2, shownrotated 90 degrees;

FIG. 13.5 is a magnified view of one end of the centraliser of FIG.13.1;

FIG. 14.1 is an isometric view of an interlocked multiple helix springcoil centraliser formed according to a further aspect;

FIG. 14.2 is a magnified view of one end of the centraliser of FIG.14.1;

FIG. 14.3 is a perspective view of the magnified view of FIG. 14.2;

FIG. 14.4 is a side view of the centraliser of FIG. 14.1;

FIG. 14.5 is an end view of the centraliser of FIG. 14.1;

FIG. 14.6 is a further side view of the centraliser of FIG. 14.4, shownrotated 90 degrees;

FIG. 14.7 shows one helix forming part of the centraliser of FIG. 14.1;

FIG. 14.8 is a side view of the helix of FIG. 14.7;

FIG. 14.9 is a side view of the helix of FIG. 14.8 shown rotated 45degrees;

FIG. 14.10 is a side view of the helix of FIG. 14.8, shown rotated 90degrees;

FIG. 15.1 is an isometric view of a multiple helix spring coilcentraliser formed according to a further aspect;

FIG. 15.2 is a magnified view of one bow spring of the centraliser ofFIG. 15.1;

FIG. 15.3 is a side view of the centraliser of FIG. 15.1;

FIG. 15.4 is an end view of the centraliser of FIG. 15.1;

FIG. 15.5 is a magnified view of one of the springs of the centraliserof FIG. 15.4;

FIG. 15.6 is a further side view of the centraliser of FIG. 15.3, shownrotated 90 degrees;

FIG. 16.1 is an isometric view of a multiple helix spring coilcentraliser formed according to a further aspect;

FIG. 16.2 is a side view of the centraliser of FIG. 16.1;

FIG. 16.3 is an end view of the centraliser of FIG. 16.1;

FIG. 16.4 is a magnified view of one of the springs of the centraliserof FIG. 16.3;

FIG. 16.5 is a further side view of the centraliser of FIG. 16.2, shownrotated 90 degrees;

FIG. 17.1 is an isometric view of a centraliser formed according to afurther aspect;

FIG. 17.2 is a magnified view of one end of the centraliser of FIG.17.1;

FIG. 17.3 is a side view of the centraliser of FIG. 17.1;

FIG. 17.4 is an end view of the centraliser of FIG. 17.1;

FIG. 17.5 is a further side view of the centraliser of FIG. 17.3, shownrotated 90 degrees;

FIG. 18.1 is an isometric view of a centraliser formed according to afurther aspect;

FIG. 18.2 is a magnified view of one end of the centraliser of FIG.18.1;

FIG. 18.3 is a side view of the centraliser of FIG. 18.1;

FIG. 18.4 is an end view of the centraliser of FIG. 18.1;

FIG. 18.5 is a further side view of the centraliser of FIG. 18.3, shownrotated 90 degrees;

FIG. 19.1 is an isometric view of a centraliser formed according to afurther aspect;

FIG. 19.2 is a magnified view of one end of the centraliser of FIG.19.1;

FIG. 19.3 is a side view of the centraliser of FIG. 19.1;

FIG. 19.4 is an end view of the centraliser of FIG. 19.1;

FIG. 19.5 is a further side view of the centraliser of FIG. 19.3, shownrotated 90 degrees;

FIG. 19.6 is an exploded view of the centraliser of FIG. 19.1;

FIG. 20.1 is an isometric view of a centraliser formed according to afurther aspect;

FIG. 20.2 is a magnified view of one end of the centraliser of FIG.20.1;

FIG. 20.3 is a perspective view of the end of FIG. 20.2;

FIG. 20.4 is a side view of the centraliser of FIG. 20.1;

FIG. 20.5 is an end view of the centraliser of FIG. 20.1;

FIG. 20.6 is a further side view of the centraliser of FIG. 20.4, shownrotated 90 degrees.

FIGS. 21.1 to 21.4 show perspective, side and end views of a centraliserformed in accordance with an aspect of the present invention;

FIGS. 22.1 to 22.5 show perspective, side and end views and a magnifiedend view of a centraliser formed in accordance with the presentinvention;

FIG. 23 shows a centraliser with a single internal stop collar, formedin accordance with the present invention and fitted onto a pipe;

FIGS. 24.1 to 24.4 show perspective, side, end and magnified end viewsof a centraliser formed in accordance with the present invention;

FIGS. 25.1 to 25.3 show perspective, side and end views of a centraliserformed in accordance with the present invention, the centraliser beingformed with a single internal stop collar and being shown fitted onto apipe;

FIGS. 26.1 and 26.2 show perspective and side views of a centraliserwith two external stop collars, the centraliser being formed inaccordance with the present invention and shown fitted onto a pipe;

FIG. 27.1 is a perspective view of a centraliser with a single internalstop collar, the centraliser being formed in accordance with the presentinvention and shown fitted onto a pipe;

FIG. 27.2 shows the centraliser of FIG. 27.1 removed from the pipe; and

FIG. 27.3 is a side view of the centraliser of FIG. 27.2.

DETAILED DESCRIPTION

Example embodiments are described below in sufficient detail to enablethose of ordinary skill in the art to embody and implement the systemsand processes herein described. It is important to understand thatembodiments can be provided in many alternate forms and should not beconstrued as limited to the examples set forth herein.

Accordingly, while embodiments can be modified in various ways and takeon various alternative forms, specific embodiments thereof are shown inthe drawings and described in detail below as examples. There is nointent to limit to the particular forms disclosed. On the contrary, allmodifications, equivalents, and alternatives falling within the scope ofthe appended claims should be included. Elements of the exampleembodiments are consistently denoted by the same reference numeralsthroughout the drawings and detailed description where appropriate.

The terminology used herein to describe embodiments is not intended tolimit the scope. The articles “a,” “an,” and “the” are singular in thatthey have a single referent, however the use of the singular form in thepresent document should not preclude the presence of more than onereferent. In other words, elements referred to in the singular cannumber one or more, unless the context clearly indicates otherwise. Itwill be further understood that the terms “comprises,” “comprising,”“includes,” and/or “including,” when used herein, specify the presenceof stated features, items, steps, operations, elements, and/orcomponents, but do not preclude the presence or addition of one or moreother features, items, steps, operations, elements, components, and/orgroups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein are to be interpreted as is customary in the art. Itwill be further understood that terms in common usage should also beinterpreted as is customary in the relevant art and not in an idealizedor overly formal sense unless expressly so defined herein.

The device 10 of FIGS. 1.1 to 1.3 aims to reduce pressure on theformation within the well by spreading the load over a greater contactarea between the centraliser and the formation wall, therefore reducingthe risk of the bows digging into the formation and thereby maintaininga more centralised location of casing, whilst reducing the effectivefriction of the contact. The larger contact area and reduced pressureresulting from the design may be particularly useful for centralisercontact with softer formations, e.g. shale, chalk, clay, etc.

The device comprises two longitudinally spaced collars 15, 20 with sixbow springs 25 connected therebetween. In other embodiments (not shown)devices may have less or more springs (for example two, three, four,five, seven, eight, nine or ten). In this embodiment two collars areprovided; in other embodiments (not shown) more spaced collars areprovided with springs therebetween.

Each of the springs 25 has an apex or peak 30 along its length. In thisembodiment the springs are wider at the peaks (to increase surface areacontact on wall formation material in use). The bow is also curved incross-section to prevent side edges of the bow from digging/gouging intothe formation circumferentially as well as longitudinally.

The lower sections of the bow are narrower to permit flexibility as thebow is acted upon, whilst the curvature enables the structure tomaintain its shape and strength. The narrow sections towards the base ofthe bows also permit a good flow-by area for cementing due to themaintaining a larger void area between bows close to the innercasing/pipe (as viewed longitudinally).

Three of the springs have their peak closer to the collar 15; three ofthe springs have their peak closer to the collar 20. This means thatalternate bow spring apexes are longitudinally offset, forming two ringsof contact where each ring is formed from three springs making threepoints of contact. There are two principal benefits: (1) the two ringsof contact present a restoring moment force onto the casing to aidcentralization, and (2) the three springs initially meeting aconstriction are compressed first and pass through the constrictionbefore the trailing three springs are compressed, the centraliser'srestoring forces would thus be better distributed longitudinally to easepassage through the constriction.

The device of FIG. 1 also aims to distribute stress better at thejunctions 35 where each bow spring is connected to the centralisedrings. The junctions 35 are rounded, which reduces the risk ofcentraliser mechanical failure.

The device 110 of FIG. 2 aims to exert more centralising force on thecasing as the compression increases.

Each spring 125 of the device has a double outward bow arrangement, witha larger, taller bow 127 and a smaller, shorter bow 129, and a singleinward bow 128 between the outward bows.

Each spring acts as a larger single and weaker bow-spring whilst subjectto minimal forces [Phase 1] (as experienced on vertical or near verticalborehole sections), but as forces are increased (due to the weight ofcasing bearing onto the centraliser in more horizontal sections) thecentral inward facing bow will connect with the casing outer wall andthe higher bow effectively transforms into a shorter, stiffer bow spring[Phase 2], and further still as the height of the two bows 127, 129become equal the system effectively transforms into two smaller andstiffer bow-springs [Phase 3]. As shown in FIG. 2.1 and FIG. 2.2:

Phase 1

RHS bow 127 only in contact with formation.

Phase 2

RHS bow 127 only in contact with formation, and central inward facingbow 128 in contact with casing.

Phase 3

RHS bow 127 and LHS Bow 129 in contact with formation, and centralinward facing bow 128 in contact with casing.

The spring therefore has multiple bows; in this embodiment being alonger, prouder bow and a shorter, less proud bow. In other embodiments(not shown) more bows may be provided, for example three or more.

An alternative fundamental design of centraliser 210 is shown in FIG. 3,which aims to provide greater contact area with the formation in orderto minimize gouging into the formation.

Up to 360 degree circumferential contact area for support (restoringforce in all radial directions) can be achieved depending on the numberof revolutions and helices employed—whilst at the same time no linearpoint contact is present that might induce gouging, i.e. the contactarea is constantly changing around the circumference of the borehole dueto the helical design. In other words there is no linear point contactin the axial (or inner pipe direction).

It induces axial tension through helical coiled springs 225, therebyinducing restoring force on the formation. Torsional forces aid passagethrough constricted sections by effectively twisting and wrappingtightly around the casing. The spiral nature of the design aids thenatural flow of the cement through and around the centralizer withminimal restriction to flow-by cross-sectional area—both importantcharacteristics to minimize the chance of unwanted voids or cavities ormud pockets. A gentle increase in slope along the path of the spiral andthe easy rotation of the helix design both aid to reduce starting andpush forces through tight spots.

The helix/spiral provides a natural guide away, and around, obstacles orprotruding rocks, and prevents snagging or sticking or lodging since thecentraliser is free to rotate around the pipe.

In FIGS. 4.1 to 4.4 a centraliser 310 formed according to an alternativeembodiment is shown. In this embodiment a single helical coiled spring325 with a circular cross section is provided. The ends 315, 320 of thespring 325 are coiled to form integral end collars. The end view (FIG.4.3) illustrates a substantially 360 degree contact whilst not being inthe same plane i.e. a single coil contacts the full circle, but thecontact is spread longitudinally. This arrangement induces axial tensionthrough helical coiled springs 325, thereby inducing restoring force onthe formation. Torsional forces aid passage through constricted sectionsby effectively twisting and wrapping tightly around the casing. Thespiral nature of the design aids the natural flow of the cement throughand around the centralizer with minimal restriction to flow-bycross-sectional area—both important characteristics to minimize thechance of unwanted voids or cavities or mud pockets. A gentle increasein slope along the path of the spiral and the easy rotation of the helixdesign both aid to reduce starting and push forces through tight spots.

FIGS. 5.1 and 5.2 show a centraliser 410 formed according to analternative embodiment. The centraliser 410 is similar to thecentraliser 310 of FIG. 4, except that the spring 425 has a generallysquare cross section.

FIGS. 6.1 and 6.2 show a centraliser 510 formed according to analternative embodiment. The centraliser 510 is similar to thecentralisers 310, 410 of FIGS. 4 and 5, except that the spring 525 has agenerally hexagonal cross section.

FIGS. 7.1 and 7.2 show a centraliser 610 formed according to analternative embodiment. The centraliser 610 is similar to thecentralisers 310, 410, 510 of FIGS. 4 to 6, except that the spring 625has a generally elliptical cross section.

FIGS. 8.1 to 8.6 show a centraliser 710 formed according to analternative embodiment.

The device 710 shares some similarities with the device 10 of FIG. 1.The highest point 730 of each bow is curved longitudinally andtransversely. In this embodiment the curvature of the wider contact areais greater.

A wide flat bow deforms in a much different way and a curvedcross-section is advantageous in terms of retaining the bow structure asit is acted upon.

The curvature has two benefits:

-   -   a) to strengthen the bows 725 and permit the use of thinner        material and/or narrower bow widths; and    -   b) it provides a ski affect in all directions, without the        chance of a sharp edge digging sideways into the formation. In        this embodiment the radius of the curvature after insertion into        the hole would be equal or less than the radius of that hole.

In this embodiment the apices 730 of all of the bows 725 are generallyin the same place longitudinally. In other embodiments (not shown) alongitudinally offset array may be provided.

In addition, in this embodiment there are rounded corners 724 where thebows 725 meet the end collars 715, 720, as shown best in FIG. 8.6.

Referring now to FIGS. 9.1 to 9.4 there is shown a centraliser 810formed according to an alternative embodiment.

The device shares some similarities with the device 10 of FIG. 1. Threeof the springs 825 a have their peak closer to the collar 815; three ofthe springs 825 b have their peak closer to the collar 820. This meansthat alternate bow spring apexes 830 a, 830 b are longitudinally offset,forming two rings of contact where each ring is formed from threesprings making three points of contact. There are two principalbenefits: (1) the two rings of contact present a restoring moment forceonto the casing to aid centralization, and (2) the three springsinitially meeting a constriction are compressed first and pass throughthe constriction before the trailing three springs are compressed, thecentraliser's restoring forces would thus be better distributedlongitudinally to ease passage through the constriction.

FIGS. 10.1 to 10.5 show a centraliser 910 formed according to analternative embodiment.

The centraliser shares some similarities with the device 710 of FIGS.8.1 to 8.6. In this embodiment each of the widened apices 930 has aplurality (in this case three, although one or more may be provided inother embodiments) of longitudinal ridges 931. The ridges reducefrictional contact on a casing in use.

FIGS. 11.1 to 11.4 show a centraliser 1010 formed according to analternative embodiment. The centraliser 1010 comprises a plurality ofgenerally straight bow springs 1025 connected at either end to arespective end collar 1015, 1020.

Each of the springs 1025 comprises a plurality of mutually spaceddimples 1026 pressed out from the inside face so that they extendradially outwards. In this embodiment the dimples 1026 are restrictedgenerally to the highest region of each bow.

The dimples 1026 help to reduce frictional contact of the bows on acasing in use whilst maintaining an overall spread of load across thefull width of the bow.

Referring now to FIGS. 12.1 to 12.4 there is shown a centraliser 1110formed according to an alternative embodiment. The centraliser 1110 issimilar to the centraliser 110 of FIG. 2 and accordingly has a pluralityof springs 1125, with each spring 125 of the device have a doubleoutward bow arrangement, with a larger, taller bow 1127 and a smaller,shorter bow 1129, and a single inward bow 1128 between the outward bows1127, 1129 to give a generally sinuous configuration.

In FIGS. 13.1 to 13.5 there is shown a centraliser 1210 formed accordingto an alternative embodiment.

The centraliser 1210 comprises multiple (in this embodiment three)coiled springs 1225 a, 1225 b, 1225 c. At the end of each spring thecoil tightens to give one complete revolution. Collectively the threeend coil termini 1227 a, 1227 b, 1227 c, 1228 a, 1228 b, 1228 c form anintegral collar 1215, 1220.

Each spring has its greatest diameter generally centrally along thelength thereof (although circumferentially offset from each other). Asshown best on FIG. 13.3, none of the coils has a full revolution at thecentre of the centraliser; all three coils are required to givesubstantially 360 degree cover i.e. enough to contact in all directions.The benefit of a multi-helix is that each helix does not have toelongate as far to wrap around a pipe.

To secure the springs together, the end coil revolutions are weldedtogether, as shown best in FIG. 13.5 in which welds 1229 adjoining thegap between coils is shown. This means that the coils cannot twistindependently of each other.

In this embodiment the welds extend along the last 120 degrees of eachhelix end i.e. there are three weld beads (in other embodiments, notshown, more or less weld beads with a great or lesser circumferentialextent may be used).

FIGS. 14.1 to 14.10 illustrate a centraliser 1310 formed according to analternative embodiment.

The centraliser 1310 is similar to the centraliser 1310 of FIG. 13, withmultiple helical coiled springs 1325 a, 1325 b, 1325 c. The ends of thesprings also together form integral end collars 1315, 1320. However, inthis embodiment there is no welding to secure the coils together.Instead, each coil terminus 1327 a, 1327 b, 1327 c, 1328 a, 1328 b, 1328c includes a kink 1322—a 90 degree return—and then continue for afurther 120 degrees with a terminal tail 1323. FIGS. 14.7 to 14.10 showone spring 1325 a separately for clarity.

Within both end collars the end of each kink butts up against the next,thus locking the springs longitudinally and circumferentially.

Referring now to FIGS. 15.1 to 15.6 there is shown a centraliser 1410formed according to a further embodiment.

The centraliser 1410 is similar to the centraliser 710 of FIG. 8 andincludes a plurality of springs 1425 each having a widened apex region1430.

In this embodiment the apex regions 1430 include a plurality of shallowscallops 1433 along both edges thereof giving a wave-like appearance.

The scallops 1433 help to reduce stresses that might otherwise build upunder load and without reducing the overall width of the increased widthapex. In this embodiment the scalloped edges extend only in the regionof the apex; in other embodiments the scallops may extend alongdifferent areas of the bows.

The stress reduction scallops are extremely useful in this embodimentbecause the apices have a curved cross section; therefore when theapices flatten and compress in use there is a need to alleviate thestresses at the outer edges of the bow.

Referring now to FIGS. 16.1 to 16.5 there is shown a centraliser 1510formed according to a further embodiment.

The centraliser 1510 is similar to the centraliser 1410 of FIG. 15 andincludes a plurality of springs 1525 each having a widened, curved(longitudinally and transversely) apex region 1530.

In this embodiment each apex region has a single, central scallop 1534along each edge. Again the scallop is provided to alleviate stressesunder loading of the springs in use without reducing the overall widthof the apex.

FIGS. 17.1 to 17.5 show a centraliser 1610 formed according to analternative embodiment. The centraliser 1610 comprises a plurality (inthis embodiment six) generally straight bow springs 1625 which merge ateach end into an end collar 1615, 1620.

This embodiment in part addresses the need to reduce the stress and theinevitable counter-acting forces resulting from an integral collar wherethe bow arch transitions to that collar i.e. as the bow flattens andelongates the end collar, being attached via a radius in the opposingdirection avoids the tendency to lift up and pivots effectively at thetransitional radius.

To avoid this the transitions 1637 between the bow springs 1625 and eachcollar 1615, 1620 are looped almost 180 degrees back underneath the archof the bow, thereby creating a pivot point where stress are minimal andfollow the same direction. In other words there is a “heel” transitionfrom the spring into the collar which allows the bows to pivot at bothends. In some embodiments the bow arch ends abruptly and pivots aboutthe end point freely, although this may not always be possible whilstmaintaining an integral all-in-one centraliser with end collars thathold it together circumferentially. This arrangement means that thecollars are created on the inside of the centraliser.

FIGS. 18.1 to 18.5 show a centraliser 1710 formed according to analternative aspect.

The centraliser 1710 comprises a plurality of generally straight bowsprings 1725 with an end collar 1715, 1720 at either end.

With the intention of reducing stress at the transition between thesprings and the collars, the flat bows 1725 are curved around (“bentback”) at both ends to form hook-like termini 1716. The end collars 1725are also provided with hook-like termini 1717 which are interspersedbetween the bow termini 1716.

A bar/ring 1718 is passed through all of the termini 1716, 1717 to jointhe bows 1725 to the collars 1715, 1720. The bows can rotate freely withrespect to the ring 1718.

The collars 1715, 1720 are again on the inside (longitudinally) of thecentraliser. In other embodiments (not shown) the collars could bepositioned on the outside (longitudinally) of the centraliser. An insidecollar may, for example, be preferred if it can be used to create a stopfor a stop collar (not shown) to hit against.

FIGS. 19.1 to 19.6 show a centraliser 1810 formed according to a furtherembodiment. The centraliser 1810 is similar to the centraliser 1710 ofFIG. 18.

In this embodiment there is no ring provided to join the springs 1825and the collars 1815, 1820. Instead, the collar is provided with sixcircumferential slots 1819 and the spring termini 1816 are curled aroundso that they thread through respective slots to firmly engage theseparate collars.

Referring now to FIGS. 20.1 to 20.6 there is shown a centraliser 1910formed according to an alternative embodiment.

The centraliser comprises a plurality of bow springs 1925 with collar1915, 1920 at either end. The transition between the springs and thecollars is designed to reduce stresses in use. A generally S-shape kink1922 is provided at the transitions to allow flexing.

As shown in the drawings, starting from the bow spring the first bend isa relatively sharp bend radially towards the centre of the inner pipe.This is followed by a 90 degree (other angles are possible) bend intothe collar. The resulting formation has two main benefits: firstly toprovide an opposing face for a single stop collar mounted centrallywithin the centraliser (as opposed to two at either end—this itself hastwo main benefits: to reduce manufacture cost; and most importantly topull the centraliser through the hole rather than push and ease passagethrough constrictions); and the S-shape transition 1922 aims to reducestress by allowing an element of pivot at the first transition.

FIGS. 21.1 to 21.4 shows a multiple offset helical spring centraliserdesign formed in accordance with the present invention.

A centraliser 2010 comprising of one or more coiled springs with aplurality of sections differing in pitch and diameter such that atransition is made from an end collar section 2001 to a centralising bowsection 2002 a to a mid-collar 2003 to another bow section 2002 b andthen to an opposite end collar 2004. There may be two or more mid-collarsections 2003 a, 2003 b (two are provided in this embodiment), and twoor more bow sections 2002 a, 2002 b, 2002 c (three are provided in thisembodiment) where each bow section is offset in angle such that theresulting peaks are evenly spaced around the circle when viewed from theend axial orientation (120 deg offset shown).

Each bow section may have partial revolution (120 deg shown) so that theaccumulative total revolution of all the bow sections is at least onefull revolution of the circle, i.e. 360 deg shown. The favoured numberof bow sections is three, each offset by 120 deg, and the favourednumber of mid-collar sections is two—this is considered the minimum toensure even force distribution on the outer casing. The favoured pitchof the end collars and mid-collars is such that the resulting coil isclosely wound, but it may be also a larger pitch to become an open coil.The diameter of the end and mid-collar sections is to suit thepipe/casing diameter so that it is free to move axially.

In this embodiment the transition from collar sections to bow sectionsis preferred to be soft and more gradual rather than a sharp and suddentransition for ease of manufacture [although the drawings show quite asudden transition—partly due to the limitations of the CAD modellingtools used to produce the sketch].

The benefit of this design is partly in the manufacturing, i.e. thelarger the pitch the more difficult it is to produce. For example it isdifficult to achieve a pitch greater than 200 mm whilst maintaining thespring properties and not lose its shape and form. Also, a fullrevolution across 200 mm pitch results in a more closely wound coil thatis not preferred ultimately due to the requirements of the centraliserto avoid obstruction downhole. For this reason it is preferred toperform a partial revolution at the same pitch, returning to amid-collar before starting a new partial revolution bow section, so longas the result as viewed axially is an even distribution of contactpoints on the outer casing wall.

It is also beneficial in function as it helps maintain spring forces byhaving a shorter pitched coil that will not stretch so easily beyond itselastic limit, as well as allowing a more gradual angle of helixentering the outer casing thereby easing its path and reducing frictionand snagging potential.

FIGS. 22.1 to 22.5 show a centraliser 2110 which addresses stressrelieving—of internally facing end collar designs.

A small radius curve/bend 2105 b between the main bow 2105 c and theinternally facing end collar 2105 d provides sufficient flexibility andmovement around the transition area 2105 a such that when the bow iscompressed and loaded the stresses are distributed through thecurve/bend 2105 b and dissipated away from the transition radius 2105 a,effectively allowing the bow to flex somewhat rather than to crease at2105 a.

FIG. 23 relates to a general discussion of a centraliser 2210 providedwith inward facing end collars 2215 a, 2215 b formed in accordance withthe present invention (for example the centraliser 2110 of FIG. 22).

Advantages of a centraliser with an inward facing end collar:

-   -   a) Only one stop collar 2206 needs to be fitted to the inner        casing/pipe 2207 rather than two. This also halves the total        quantity to be manufactured and bought, making it cheaper,        quicker and less to transport. Halving the effort and time        required to install stop collars by the rig site operators is a        major advantage.    -   b) Another major functional advantage is that the stop collar        effectively ‘pulls’ the centraliser down the hole rather than        ‘pushing’. This is mechanically beneficial and preferred as it        helps minimise the chance of sticking/jamming, as the bows are        allowed to flex/bend away from the point of force applied        axially and so does not need to fight against it. The        centraliser does not need to elongate in the opposing direction        to the applied force, rather with it. An analogy might be made        with pulling a string through a tube rather than fighting to        push it through, or dragging a branch across the ground by the        base rather than by the tips.

Disadvantage:

-   -   c) A disadvantage might be considered to be that the centraliser        is restricted in its ability to compress as close to the inner        pipe surface as with other traditional bow spring centralisers.        In theory this is possibly a concern on really tightly        constricted holes, but in reality/practice this may rarely be an        issue as the well bore diameter should never be this        constricted.

FIGS. 24.1 to 24.4 show a centraliser 2310 which address stressrelieving—of externally facing end collar designs.

A small radius curve/bend 2305 b between the main bow 2305 c and theexternally facing end collar 2305 d provides sufficient flexibility andmovement around the transition area 2305 a such that when the bow iscompressed and loaded the stresses are distributed through thecurve/bend 2305 b and dissipated away from the transition radius 2305 a,effectively allowing the bow to flex somewhat rather than to crease at2305 a.

This reduces the tendency for the end collar to rise (or at least tryto) as it counter acts the forces applied on the bow.

Further still the fillet 2305 e adjoining the bow to the end collar isstrictly kept well past (on the collar side) the curve 2305 b so as toensure the forces acting through the curve are kept in the same plane,which further reduces the material stresses in general as they are notcombatting the cross sectional curvature of the collar. In other wordsthe bow and the stress relieving curve at its heel are all rectangularin cross section rather than a complex 3D curved shape. This means thatthere is no requirement for the material to flex circumferentially atthe same time that it might be required to flex axially or radially,i.e. stress points and fatigue points are much reduced.

FIGS. 25.1 to 25.3 show a centraliser 2410 with a double-acting/twinphase bow design (with inward facing collars).

This embodiment originated as an exaggerated stress relieving curve,this embodiment has a curve/bump 2408 directly under the main bow 2409effectively acting as a secondary bow spring—actually there are two, oneat either each end of the main bow. They lead into an inward facing endcollar, but in addition there is a radially support collar 2408 aimmediately after the heel of the main bow and before the secondarycurve 2408. This is important in maintaining the overall shape of thecentraliser and preventing distortion, breaking, entanglement, gouging,twisting, etc. of the ends of the main bow as it passes through thehole.

2408 b is effectively the end collar that contacts with the stop collar2406. 2408 a is a secondary collar that ensures no distortion/lift/riseoccurs during passage through the wellbore (hitting rocks etc.). Bothsections 2408 a and 2408 b are rings that hold the bows together aroundthe pipe, but it is only 2408 b that meets with the stop collar.

In a similar function to that of the three phase triple bow of FIG. 2,this twin phase is beneficial in providing a minimal centralising forcein vertical sections of the well before adapting to a greater restoringforce functionality as the main bow compresses into contact with thesecondary, smaller bows underneath, which are stiffer due to their sizeas well as the fact that there are two of them working in tandem. Thisadditional restoring force is required in more horizontal sections ofthe well when the weight of the pipe and cement becomes a factor.

FIGS. 26.1 and 26.2 illustrate a centraliser 2510 with a wavy orscalloped end collar. The end collar design is applicable to otheraspects and embodiments of the present invention.

The end collars 2511 a and 2511 b are profiled to form a wave orscalloped edge so that the peak of each wave is the point of contactbetween the end collar 2511 b and the stop collar 2512 b and so the areaof contact is minimal. This reduces friction between the two opposingfaces allowing the centraliser 2510 to spin around the inner casing/pipe2513 more freely. This is advantageous to the well drilling whenrotating the casing so as to avoid accumulative forces from manycentralisers.

FIGS. 27.1 to 27.3 show a centraliser 2610 having an end collar withinward facing castellations.

End collars 2614 a, 2614 b are profiled so that the portion between eachbow 2625 around the circumference 2615 is elongated inwards to meet witha single stop collar 2616 situated internally within in the centraliserbetween the two opposing end collars 2614 a, 2614 b.

This provides the functionality of the single stop collar (i.e. pullingthe centraliser rather than pushing, as well as only needing one ratherthan two), whilst presenting a potentially easier manufacture techniqueof the more traditional end collar (i.e. the heel of the bow 2617 is notdoubled back in on itself (as in some other claims with single internalstop collars), which might present difficulties in manufacturing from asingle sheet, as well as making the height from the casing greater atthe heel when compressed. In other words this castellated design canflatten against the inner casing 2618 with a height equal to thematerial thickness only.

The profile of the contact face between the castellation 2615 and thestop collar 2616 is drawn here as a flat or square face, but this couldequally be rounded or scalloped or wavy so as to provide minimal contactwith the stop collar (as described in relation to FIG. 26), with thebonus of less friction when casing is rotated.

In any of the aspects and embodiments of the present invention there maybe consideration given to reduction of co-efficient of friction oncentraliser surfaces in contact with the formation as well as betweenbow-spring ring/collar and the casing outer wall (bow-spring needs torevolve around the casing to allow casing rotation) through use ofeither low friction coatings, low friction pads or roller devices.Examples of suitable low friction coatings are: Molybdenum Disulphide,Diamond Like Carbon (DLC), which would be applied as thin coatings thatwould flex with the bow spring during its operation.

Another way to reduce the effective friction between the bow spring andthe casing or formation wall is with the introduction of surfacetextures to reduce the immediate contact area with the casing/formation,whilst at the same time maintain the overall spread of load across thebow. Such surface textures might consist of dimples, or dome shapeimpressions, or longitudinal ridges.

Although illustrative embodiments of the invention have been disclosedin detail herein, with reference to the accompanying drawings, it isunderstood that the invention is not limited to the precise embodimentshown and that various changes and modifications can be affected thereinby one skilled in the art without departing from the scope of theinvention as defined by the appended claims and their equivalents.

1-64. (canceled)
 65. A centraliser comprising longitudinally spacedcollars connected by a plurality of springs, each of the springscomprising two or more bow sections.
 66. A centraliser as claimed inclaim 65, in which each spring comprises a larger, taller outward bowsection and a smaller, shorter outward bow section, and an intermediateinward bow section between the outward bows.
 67. A centraliser asclaimed in claim 65, in which the bow sections have a different heightin a resting position.
 68. A centraliser as claimed in claim 65, inwhich each spring has a region of increased width.
 69. A centraliser asclaimed in claim 68, in which the cross section of the region ofincreased width is curved.
 70. A centraliser as claimed in claim 65, inwhich the spring contacts the collars at junctions, and in which thejunctions are rounded whereby to reduce stress.
 71. A centraliser asclaimed in claim 65, in which the longitudinal position of the outwardbow section peaks on at least two springs is different.
 72. Acentraliser as claimed in claim 71, in which alternate springs havedifferent peak positions.
 73. A centraliser as claimed in claim 71, inwhich there are two spring section peak positions which arelongitudinally offset from each other.
 74. A centraliser as claimed inclaim 65, in which at least part of the centraliser has surfaceformations.
 75. A centraliser as claimed in claim 74, in which thesurface formations are protruding dimples.
 76. A centraliser as claimedin claim 65, in which at least part of the centraliser has alow-friction coating.
 77. A centraliser as claimed in claim 76, in whichthe coating is Molybdenum Disulphide or Diamond Like Carbon.
 78. Acentraliser as claimed in claim 65, in which the centraliser is providedwith one or more low-friction pads.
 79. A centraliser as claimed inclaim 65, in which the centraliser is provided with one or more wearpads.
 80. A centraliser as claimed in claim 65, in which each bow hasscalloped or wavy or ribbed edges for reducing the stress incurred asthe bow flexes in use, whilst at the same time maintaining the overallcontact area for skiing across a formation.
 81. A three phasecentraliser for preventing a casing from contacting a wellbore wall, thecentraliser comprising a plurality of springs, each spring comprising adouble outward bow section arrangement and an inward bow section betweenthe outward bow sections, the outward bow arrangement comprising alarger, taller bow section and a smaller, shorter bow section, in which:in a first phase, each spring acts as a larger single and weakerbow-spring whilst subject to minimal forces; in a second phase, asforces are increased, the inward facing bow contacts with the casing andthe double outward bow arrangement transforms into a shorter, stifferbow spring; and in a third phase, as the height of the two outward bowsections become generally equal, the springs transform into two smallerand stiffer bow-springs.
 82. A casing fitted with one or morecentralisers as claimed in claim
 65. 83. A well bore having one or morecentralisers as claimed in claim 65.